Transcript NVIS - RAYNET

NVIS
NVIS
 What is NVIS ?
 Means Near-Vertical Incidence Skywave
 Opposite of DX (long – distance)
 Local - to - Medium Distance (0 – 250 mls)
‘Ordinary’ Propagation
To travel a long distance, the signal must
take off at a LOW angle from the antenna
– 30 degrees or less
This is so that it can travel the maximum
distance before it first arrives at the Ionosphere
Long gap before signal returns to earth – the
part in between this and the end of the ground
wave is the so-called Skip (or Dead) Zone
‘Ordinary’ Propagation
Illustration courtesy of Barrett Communications Pty
NVIS Propagation
To travel a local - medium distance, the signal
must take off at a HIGH angle from the antenna
– typically 60 – 90 degrees
This returns from the Ionosphere at a similar
angle, covering 0 – 250 mls
It thus fills in the Skip (or Dead) Zone – like
taking a hose and spraying it into an umbrella !
NVIS Propagation
Illustration courtesy of Barrett Communications Pty
Using NVIS successfully
HIGH angle of radiation from antenna
Minimise ground wave, as it will interfere
with the returning skywave
Most importantly, CHOOSE THE
CORRECT FREQUENCY BAND – go too
high in frequency and your signal will pass
through straight into space!
Choosing the right frequency
The Ionosphere – D, E, F1 & F2 layers
D and to a lesser extent, E layers attenuate and
absorb signal
Best returns from F2 layer
At any one time we need to know the frequency
of the F2 layer – The Critical Frequency or foF2
Optimum frequency for NVIS work around 10%
below this
The Ionosphere
Illustration courtesy of the University of Ulster Communications Centre
NVIS - Frequency and Time
In practice, highest NVIS frequency can reach
10 MHz band. Lowest can go down to 1.81
MHz band
‘Higher’ frequency band during day, ‘Middle’
frequencies afternoon/evening, ‘Lower’
frequencies at night
Frequencies also affected by time of year and
period of sunspot cycle
For best results, these three different frequency
‘bands’ required
NVIS – The Critical Frequency
The Critical Frequency is the key to successful
NVIS working
The Critical Frequency (or foF2) is the highest
frequency at any one time that a signal
transmitted vertically will be returned to earth.
Anything above this passes into Space
As we are interested in vertical signals for
NVIS, then the value of the Critical Frequency
(foF2) at any one time is of great importance to
us
How can we find or estimate foF2 ?
NVIS – Finding The Critical Frequency
Real-time web information from
Ionosondes
Websites offering Critical Frequency
predictions: – IPS foF2 World Maps
Software Propagation prediction tables or
similar printed material: - W6ELprop,
VoACap etc.
Rule-of-thumb:- ‘higher’ band by day,
‘middle’ band afternoon/evening transition,
‘lower’ band nightime
Interpreting an Ionogram
Real –Time Ionogram
IPS foF2 World Map
Courtesy of RAL Short Term Ionospheric Forecasting Site
NVIS – For the Radio Amateur
In practice, 7 Mhz (40m) usually ‘highest’ band
3.5 MHz (80m) next lowest
1.81 MHz (160m, ‘Topband’) the lowest
80m and 160m strongly affected during the day
by absorption from the D-layer, plus noise at
night and varying times of the year
Hence the need for a ‘middle’ transition
frequency around 5 MHz and why Amateur
Radio is seeking frequencies in this area of
the spectrum
NVIS – The Antenna Side
Need high angle (60-90°) radiation for NVIS
Verticals are no use – predominantly low angle
A Half wave dipole at ‘text book’ height – 0.5
wavelength produces low angle radiation, BUT, if
lowered to 0.25 wavelength or below, produces
high angle radiation !
Not too low, though – some earth losses. A
reflector wire or earth mat can reduce this
A Vertical = No High Angle Radiation
Courtesy of ARRL Handbook
A Horizontal dipole
at ‘textbook’ height
Textbooks say that for a horizontal dipole
to radiate low angle radiation, it must be
half (0.5) a wavelength above ground
In the case of the lower bands such as 80
and 160m, this would be pretty high!
A Horizontal dipole at ‘textbook’ height
A Low Horizontal dipole = High Angle
If the height of the dipole is lowered, the angle of
radiation becomes higher and the low angle
radiation starts to disappear
The optimum amount of high angle radiation is
obtained at a quarter- (0.25) wavelength above
ground
Going lower than 0.25 causes efficiency loss
In practice 0.25 – 0.15 wavelength heights used
for NVIS
A Low Horizontal dipole = High Angle
Illustration courtesy of NVIS Communications (Worldradio Books)
NVIS – Monoband Antennas
The dipole is essentially a single band antenna
There are also a couple of special higher-gain
single band NVIS antennas –
Dipole with reflector
The Shirley
The Jamaica
The G8ATH Inwardly Inclined Dual Monopole
(or IIDM)
NVIS – Dipole with Reflector
Illustration courtesy of NVIS Communications (Worldradio Books)
NVIS – The Shirley Antenna
Illustration courtesy of NVIS Communications (Worldradio Books)
NVIS – The Jamaica Antenna
Illustration courtesy of NVIS Communications (Worldradio Books)
NVIS – The G8ATH IIDM Antenna
NVIS – Multiband Antennas
As mentioned earlier, at least three different
frequency bands are needed for successful 24
hour NVIS operation and so multi or wideband
antennas are used
Simple ones include long wire, inverted-L,
Shallow (120°) Inverted-Vee Doublet with open
feeder, full-wave low (0.15-0.25λ) horizontal loop
(reflector could also be used below this)
Other multiband antennas can be used -
NVIS – The Fan Dipole
Illustration courtesy of NVIS Communications (Worldradio Books)
NVIS – The AS2259 or ‘Collins’ Antenna
Illustration courtesy of NVIS Communications (Worldradio Books)
NVIS – The Jumpered Doublet
Illustration courtesy of NVIS Communications (Worldradio Books)
NVIS –Wideband Folded Dipole (T2FD)
Antenna total length approx 90ft
600 Ω Terminating Resistance/Balancing Network
12 : 1 Stepdown Balun to 50 Ω
Example – Barker & Williamson BWD 1.8 – 30 MHz Wideband Folded Dipole
Courtesy of Barker & Williamson Manufacturing Inc.
NVIS – Broadcast Log-Periodic
ABC
(Australian Broadcasting Corporation)
Alice Springs
NVIS Transmitter Site
TCI 615
Log-Periodic
Antenna
NVIS – Mobile Operation
You can use a whip for NVIS – but NOT
VERTICAL ! You can either
a) Bend the whip back over the vehicle as flat as
possible without breaking (see Military on TV)
b) Bend the whip back away from the vehicle at
least 45°- OK when stationary, but not
recommended mobile ! - Keep your distance !
You can use loops –
Magnetic Loops are the most favoured
Take care as high RF voltages exist on certain
parts of these antennas
NVIS – Tilt Angle Adaptor
Illustration courtesy of NVIS Communication – Worldradio Books
NVIS – Codan’s Whip Method
Illustration courtesy of Codan Pty.
NVIS – G8ATH IIDM used mobile
NVIS – The Magnetic Loop : Theory
NVIS – The Magnetic Loop : Theory
The Loop is very small compared to wavelength
Very High ‘Q’ = narrow bandwidth (can improve
signal-to-noise ratio)
Resistive losses must be kept as low as possible
High Voltage and Current in loop – Tens of Amps
and quite a few kV = special care needed !!
High voltage tuning capacitor needed – usually
Vacuum Variable Capacitor required ; they’re not
particularly cheap !!
Predominant radiation upwards
NVIS – Vacuum Variable Capacitors
Glass
Ceramic
NVIS – Magnetic Loop Modelling
NVIS – The Magnetic Loop (Dubai Style !)
NVIS – The Magnetic Loop ( UK Style ! )
NVIS – The Magnetic Loop (Aussie Style !)
NVIS – The Magnetic Loop (French Style !)
NVIS – The Magnetic Loop (Russian Style !)
Photo PA3EQB
NVIS – The Magnetic Loop
(O.T.T. or Clothes Rack Style ?)
Photo WB3AKD
NVIS - A few other aspects….
NVIS in WW II
For D-Day : Successful communications
between Operations HQ at Uxbridge, forward
control ship USS Ancon and landing parties
achieved using horizontal antennas and highangle skywave, following poor results with
verticals – done by Dr. Harold Beverage (of long
antenna fame !)
Germans also used NVIS Mobile antennas in
WW II
 ‘Tone’ Burst’s view of NVIS !
NVIS on D-Day
Illustration courtesy of NVIS Communication, Worldradio Books
WWII German Radio Vehicle with NVIS Antenna
(Illustration)
WWII German Radio Vehicle with NVIS Antenna
(Operational Desert Photo)
Photo Schiffer Publishing/Tactical Link
‘Tone’ Burst’s View of NVIS
RSGB Radio Today Sept 2000
ALE : Automatic Link Establishment
ALE scans and tests sets of frequencies –
usually in several bands - for a particular path or
net until it finds a frequency that will support
communications over the path.
Each radio in an ALE net constantly broadcasts
a sounding signal and “listens” for other
sounding signals generated by other net
members
Analysis of these signals by processing
determines the best frequency for
communication at the time and this frequency is
then selected automatically for operations
G4GUO’s ALE Controller Programme for PCs
Charles Brain, G4GUO
Useful websites connected with NVIS
 http://www.ukssdc.ac.uk/ionosondes/view_latest.html Chilton, UK Ionosonde –the
nearest Real-Time Ionogram for foF2 Critical Frequency
 http://digisonde.oma.be/ Ionosonde at Dourbes, Belgium (next nearest to UK)
 http://www.ips.gov.au Australian Space Weather agency. Several useful maps.
Covers Europe
 http://www.codan.com.au/ Codan Communications (Australia) HF SSB &
Satellite
 http://www.barrettcommunications.com.au/ Barrett Communications ( Australia )
Commercial HF SSB
 http://www.hflink.com Worldwide Amateur HF ALE Network – includes NVIS
 http://raynet-hf.net RAYNET HF Team – lots of useful information
NVIS - Summary
 Covers 0 – 250 mls using High-Angle (60-90°) Skywave
 Choice of Correct Frequency Band just below the Critical
Frequency is most important.
 Antenna must be horizontal, not vertical (with the
exception of magnetic loops)
 Antenna must be low – between 0.25 and 0.15 of a
wavelength above ground
 An NVIS antenna has omnidirectional radiation
 Multiband antenna (at least three bands) needed for
24hr NVIS coverage
BJ Skips, Wigan
NVIS
Near-Vertical
Incidence
Skywave
Text & Graphics by Paul Gaskell, G4MWO
Rev 04b 1st Dec 2011
First prepared Summer 2002